An emergency lighting device, in the context of the present invention, is a device which is capable of energizing a lamp when the power to the usual illumination equipment fails. Usually, such devices include a battery, some form of voltage sensing device and a separate lamp, not necessarily part of the normal illumination system, which is electrically connected to the battery only when normal power fails or, in some cases, for test purposes.
In recent years, more attention has been directed to devices which have characteristics making them suitable for use in lighting systems which employ fluorescent and other gas discharge lamps. Additionally, it has been recognized that it is practical to use some of the normal illumination lamps for emergency lighting by fitting selected fluorescent fixtures with special circuits and batteries for operating those fixtures from normal power when it is available and from the batteries when normal power fails.
Examples of circuits for performing these kinds of functions are shown in the following U.S. Pat. Nos.: 3,833,817 to Patel; 4,030,013 to Watrous; 4,216,410 to Feldstein; 4,323,820 to Teich; 4,454,452 to Feldstein.
A review of these reveals that the circuits disclosed therein are generally quite complicated and are also rather expensive to produce because of the relatively large number of components required and the nature of those components. It is also apparent that these circuits do not make most efficient use of battery power, thereby shortening the time emergency illumination can be supplied. This can be a serious shortcoming in the event of an extended power failure.